High strength polyamide yarns are known and are for example applied in tire cords, air bags. The yarns may be prepared from for example polyamide-6 or polyamide-66. There is a continuous demand for providing yarns with higher strengths, for which tenacity values is a measure to define strength. Fiber tenacity can be measured, e.g. by a STATIMAT 4U automatic tensile testing machine, according to ASTM D885-04. By having higher strength yarn, the total weight of the end product can be reduced in order to keep the same product strength, which is beneficial from an environmental perspective. For example, in the automotive industry, more and more air bags are being employed, which require high strength yarns. By increasing the strength of the yarn, the total weight of air bags can be reduced, which results in lower CO2-emissions for the car. Alternatively, other applications become feasible for high strength yarns, in which strength is a key factor, such as ropes, tire cords, etc.
Many attempts have been made to increase strength of a yarn, which includes for example the preparation of yarns from thin filaments and utilizing special cooling equipment and steam treatment, as disclosed in EP2264235. EP2264235 discloses yarn for air bags comprising polyamide, in which the single fiber fineness is 1 to 2 dtex and in which strengths are reached of around 8.5 cN/dtex.
Another example in the prior art, disclosed in U.S. Pat. No. 4,701,377, allows preparing fibers stronger than 12 g/d (ca 10.5 cN/dtex) following a special spinning and drawing procedure, including superheated steam treatment.
The main disadvantage about the methods disclosed, however, is that special non-standard equipment is necessary to reach superior properties, which renders all these methods economically less attractive. Commercially available polyamide 6 and polyamide 66 yarns, produced with economically feasible processes, feature tenacity of no more than about 8.5 cN/dtex.
It is an object of the present invention to provide yarns which exhibit higher strengths and may be produced in an economically feasible way.
This object has been met by a yarn comprising a copolyamide in an amount of at least 90 wt % with respect to the total weight of the yarn, which copolyamide comprises                a) at least 95 wt % by weight with respect to the total weight of copolyamide, monomeric units derived from hexamethylene diamine and adipic acid and        b1) cyclic monomeric units derived from a diamine X, and cyclic monomeric units derived from a diacid Y, and/or        b2) cyclic monomeric units derived from an amino acid Z,in which the summed amount of monomeric units derived from X,Y and Z is between 0.1 to 4.5 wt % with respect to the total weight of the copolyamide and wherein the yarn has a tensile strength of at least 80 cN/tex as measured according to ASTM D885-04.        